Sweet Potato, Kale and Mushroom Gratin (Blue Apron)

¾ Cup Milk
6 Ounces Cremini Mushrooms
3 Cloves Garlic
2 Sweet Potatoes
1 Apple
1 Bunch Kale
1 Head Butter Lettuce
1 Lemon
3 Tablespoons All-Purpose Flour
2 Tablespoons Butter
2 Tablespoons Crumbled Blue Cheese
2 Tablespoons Roasted Pistachios
¼ Cup Grated Parmesan Cheese

Preheat the oven to 450°F. Wash and dry the fresh produce. Heat a medium pot of salted water to boiling on high. Thinly slice the mushrooms. Remove and discard the kale stems; thinly slice the leaves. Peel the sweet potatoes; thinly slice into rounds. Using a peeler, remove the yellow lemon rind, avoiding the white pith; mince the rind to get 2 teaspoons of zest (or use a zester). Quarter and deseed the lemon. Roughly chop the pistachios. Peel and mince the garlic. Cut off and discard the lettuce root. Core and thinly slice the apple; place in a bowl with the juice of 1 lemon wedge.

In a large pan (nonstick, if you have one), heat 2 teaspoons of olive oil on medium-high until hot. Add the mushrooms and cook, stirring occasionally, 4 to 6 minutes, or until lightly browned and softened. Season with salt and pepper. Add the kale and cook, stirring occasionally, 2 to 3 minutes, or until wilted. Add ¼ cup of water; cook, stirring occasionally, 2 to 4 minutes, or until the water has cooked off. Turn off the heat; stir in the lemon zest. Season with salt and pepper to taste. Transfer to a large bowl and set aside in a warm place.

While the vegetables cook, add the sweet potatoes to the pot of boiling water and cook 12 to 14 minutes, or until tender when pierced with a fork. Drain thoroughly; transfer to a plate and set aside in a warm place. Rinse and wipe out the pot.

In the same pot, heat the butter on medium-high until melted. Add the garlic; season with salt and pepper. Cook, stirring occasionally, 30 seconds to 1 minute, or until fragrant. Add the flour and cook, whisking frequently, 30 seconds to 1 minute, or until thoroughly combined. Slowly whisk in the milk and ½ cup of water; cook, whisking frequently, 2 to 4 minutes, or until thickened. Remove from heat and season with salt and pepper to taste.

Spread about ⅓ cup of the sauce into the bottom of a baking dish. Evenly top with ⅓ of the cooked sweet potatoes, then ⅓ of the cooked vegetables, then ⅓ of the remaining sauce. Repeat twice with the remaining sweet potatoes, cooked vegetables and sauce. Evenly top with the Parmesan cheese. Bake 14 to 16 minutes, or until golden brown and heated through. Remove from the oven. Let stand for at least 2 minutes before serving.

While the gratin bakes, squeeze the juice of the remaining lemon wedges into a bowl. Slowly whisk in 2 tablespoons of olive oil until well combined; season with salt and pepper to taste. Just before serving, in a large bowl, combine the lettuce, apple, blue cheese, pistachios and enough of the dressing to coat the salad (you may have extra). Toss to combine; season with salt and pepper to taste. Serve the baked gratin with the salad on the side.